4.1: Biologists Use Microscopes and Biochemistry to Study Cells

Microscopy

Microscopy is essential for studying cells, allowing scientists to observe structures beyond the capability of the human senses. Advances in microscopy have enabled the visualization of cellular components and organelles.

• Robert Hooke first viewed cells; Anton van Leeuwenhoek viewed animalcules with specialized lenses.

• Light microscopes bend light to magnify images. Key features include magnification (ratio of image size to real size), resolution (clarity of image), and contrast (difference in brightness between areas).

• Most cells are between 1 and 100 μm in size.

• Resolution barrier prevented study of organelles until the development of the electron microscope (EM), which has a resolution of about 0.002 μm.

• SEM (Scanning Electron Microscope): Used for detailed study of specimen topography. Electrons scan the surface, and the device translates electron patterns into an electronic signal.

• TEM (Transmission Electron Microscope): Used to study internal cell structure. Electrons pass through thin sections, revealing cellular structures.

• Both SEM and TEM use electromagnets; electron microscopy kills cells in preparation.

• Super-resolution microscopy allows researchers to distinguish subcellular structures as small as 10–20 μm.

• Cryo-electron microscopy preserves specimens at extremely low temperatures, allowing cells to be viewed in their environment.

Cell Fractionation

Cell fractionation separates cell components by centrifugation, allowing researchers to study specific cell parts and their functions.

• Cell components settle at the bottom of the tube, forming a pellet.

• Researchers can prepare specific cell components in bulk and identify their functions.

4.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure and organization. This distinction is fundamental in biology.

• Prokaryotes: Bacteria and archaea

• Eukaryotes: Protists, fungi, animals, plants

Comparing Prokaryotic and Eukaryotic Cells

• All cells share plasma membrane, cytosol, chromosomes, and ribosomes.

• Major difference: location of DNA. Eukaryotic DNA is found in the nucleus; prokaryotic DNA is in the nucleoid.

• Cytoplasm in eukaryotes is the region between the nucleus and plasma membrane, containing organelles. Prokaryotes lack membrane-bound organelles.

• Prokaryotic cytoplasm is not a form-filled compartment; some structures are surrounded by proteins, not membranes.

• Mycoplasmas are the smallest cells capable of programming metabolism and cellular activities.

• Plasma membrane (PM) is a selective barrier allowing passage of oxygen, nutrients, and waste.

• Surface area to volume ratio is critical: as cell size increases, volume grows faster than surface area, limiting material exchange.

• Smaller objects have a greater ratio of surface area to volume.

• Extra cellular matrix holds cells together; larger organisms do not have larger cells, just more of them.

• High surface area to volume ratio is important for cells exchanging materials with their surroundings (e.g., intestinal cells with microvilli).

4.3: The Nucleus and Ribosomes

The Nucleus: Information Central

The nucleus contains most of the cell's genes and is surrounded by a double membrane called the nuclear envelope.

• Nuclear envelope: Double membrane (lipid bilayer) perforated by pore structures.

• Pore complex: Regulates entrance and exit of proteins, RNAs, and macromolecules.

• Nuclear lamina: Protein network supporting the envelope.

• DNA is organized in chromosomes, each containing one long DNA molecule associated with proteins (histones).

• Complex of DNA and proteins is called chromatin.

• When not dividing, chromatin appears as a diffuse mass; chromosomes are indistinguishable. During division, chromosomes condense and become visible.

• Each eukaryotic species has a characteristic number of chromosomes (humans: 46; sex cells: 23).

• Nucleolus: Site of rRNA synthesis and ribosome assembly.

• Ribosomal subunits exit the nucleus via nuclear pores and assemble in the cytoplasm.

• There can be two or more nucleoli.

Ribosomes: Protein Synthesis

• Ribosomes build proteins in two locations: free in cytosol and bound to ER or nuclear envelope.

• Free and bound ribosomes are structurally identical; most proteins made by free ribosomes function in the cytosol, while bound ribosomes make proteins for membranes or secretion.

• Example: Pancreatic cells secrete digestive enzymes made by bound ribosomes.

4.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions

Endomembrane System Overview

The endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane. It is responsible for synthesis, transport, and modification of proteins and lipids, as well as detoxification.

• Membranes are related by direct physical continuity or by transfer of membrane segments as vesicles.

• Membranes are not identical in structure and function.

The Endoplasmic Reticulum (ER): Biosynthetic Factory

• ER is an extensive network of membranes accounting for more than half the total membrane in many eukaryotic cells.

• Consists of cisternae (network of membranous tubules and sacs); lumen is the internal compartment.

• ER membrane separates the internal compartment from the cytosol.

Functions of Smooth ER

• Lacks ribosomes.

• Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons, stores calcium ions.

• Cells that synthesize and secrete steroid hormones have abundant smooth ER.

• Pumps calcium ions from cytosol into ER lumen.

Functions of Rough ER

• Has ribosomes attached.

• Synthesizes proteins and secretes them.

• Polypeptide chain grows from bound ribosome, threaded into ER lumen through a protein complex.

• Most secretory proteins are glycoproteins (proteins covalently bonded to carbohydrates).

• Secretory proteins are transported in vesicles from one part of the cell to another.

• Membrane factory: adds membrane proteins and phospholipids to its own membrane.

• Makes membrane phospholipids from precursors in cytosol.

The Golgi Apparatus: Shipping and Receiving Center

• Consists of stacks of flattened membranous sacs called cisternae.

• Structural and functional directionality: cis face (receiving side) is located near the ER; trans face (shipping side) dispatches vesicles to other destinations.

• Acts as a warehouse: receives, sorts, modifies, ships, and manufactures products.

• Modifies proteins and products; glycoproteins have their carbohydrate chains altered (removing and adding sugars).

• Membrane phospholipids can also be modified.

• Produces some polysaccharides.

Additional info:

• Formulas: Surface area to volume ratio for a sphere: ,

• Example: Intestinal cells have microvilli to increase surface area for absorption without increasing volume.

• Key Terms: Selective permeability, peripheral proteins, transport proteins, glycoproteins, concentration gradient, passive transport, facilitated diffusion, active transport, tonicity, membrane potential, sodium-potassium pump, endocytosis, exocytosis.